linux-kernel - Re: Is there a notification mechanism for enabled/disabled trace events?

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Message-ID: <CAAVeFuKzain=pJ7dpKWnZyaCa9BncTXOO2CM-YwS=9jCC466-Q@mail.gmail.com>
Date:	Sun, 10 Nov 2013 13:17:09 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Is there a notification mechanism for enabled/disabled trace events?

Hi Steven, thanks for your reply!

On Thu, Nov 7, 2013 at 11:04 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Thu, 7 Nov 2013 17:42:54 +0900
> Alexandre Courbot <gnurou@...il.com> wrote:
>
>> Hi everyone,
>>
>> Trace events can be enabled through debugfs by e.g. writing '1' into
>> their enable node. This is a very useful feature as some tracing
>> functions can introduce overhead and we only want them active when
>> needed.
>>
>> There is one additional thing that I would need though, which is to be
>> notified when a given trace event is enabled or disabled.
>>
>> Here is why: I have a power monitoring hardware that can report how
>> much power is currently used by the system. Having this information
>> correlated with other traces (cpufreq, cpuidle, ...) is very useful ;
>> it can be done by repeatedly scheduling a work that probes the power
>> usage and traces it. The job should only be running when the power
>> monitoring trace event is enabled, but AFAIK there is no way to be
>> notified when it is enabled or disabled. So here are a few questions:
>
> I'm a little confused by this. Your power monitoring is only enabled
> when someone enables the power tracepoints? Why not have the user start
> monitoring and have it enable the tracepoints itself?

Mostly, for convenience reasons. There are quite a few user-space
tools that configure and control kernel tracing more conveniently,
using the trace events debugfs interface. Having a separate control to
enable current probing would add one step to the setup, and I wondered
if I could not reuse what exists already.

>> 1) Is there such a notification mechanism for trace events that I have missed?
>
> That you missed? Can you elaborate here.

I was just wondering if what I was looking for already existed - which
seems to be the case.

>> 2) If not, is there any objection to having one? I'd say my use-case
>> is not so uncommon and others would certainly benefit from it.
>>
>> 3) What would be the right place to have it? ftrace_event_reg() looks
>> like a good place to call a notifier chain, however I'm not sure where
>> the notifier head should be stored, due to my poor understanding of
>> ftrace.
>
> There is a way to hard code a notifier for tracepoints, look at how
> TRACE_EVENT_FN() is used (include/trace/events/syscalls.h)
>
> But having a generic notifier may not be too hard or invasive to
> implement.

TRACE_EVENT_FN() is actually what I was looking for! Using it and a
few registration functions for drivers to be notified when the event's
status changes, it is quite easy to achieve what I wanted.

With that in mind, it is probably not worth to make the trace events
bigger by adding a generic notification mechanism to them, unless more
potential users show up.

Thanks!
Alex.
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